This invention concerns in general and in a first of its aspects, the chemical industry and, in particular, a method for high-yield recovery and purification of ethylene as well as other products originating from a gas produced by pyrolysis of hydrocarbons. This invention also concerns an installation and equipment for exploiting this method on an industrial scale.
A large number of papers and patents addressing the production, recovery, and purification of olefins show their industrial importance and the problems encountered in the exploitation of the various processes.
Recently, the production capacity of ethylene units has attained and even exceeded the level of 1 million tons per year for a single line; which requires a new approach in the design of the process, equipment, and the controllability of the unit.
In systems of recovery and purification, particularly for ethylene, the elimination of acetylene is a key element in purification. Because of its relative volatility with respect to ethylene and ethane, it cannot be separated by distillation. In industrial practice, only two processes are applied: absorption of acetylene by a solvent and hydrogenation to ethylene and ethane.
The first method involves the use of a solvent which is usually N,N-dimethylformamide (DMF) or N-methylpyrrolidone (NMP), which allows for preferential recovery of dissolved acetylene.
The second method, catalytic hydrogenation, is generally carried out by treatment of all the gas from cracking before separation of the hydrogen contained in it, or a separate treatment of the cuts containing C2 hydrocarbons after addition of sufficiently pure hydrogen to transform all the acetylene into ethylene and ethane. These two types of hydrogenation use palladium-based catalysts with different formulations.
The stage of hydrogenation of acetylene has also been the subject of a number of papers and inventions dealing with the catalyst system and the formulations of the catalyst, and exposing the specific disadvantages connected with each of the hydrogenation technologies.
Thus, in the case of treatment of all the cracking gas originating from the pyrolysis of hydrocarbons in a hydrogenation reactor, a racing reaction may occur, corresponding to an acceleration of the kinetics of the reaction transforming the acetylene into ethylene (and also undesirable secondary reactions) because of a significant increase in the temperature of the catalyst together with the presence of a large excess of hydrogen (50 to 100 times the quantity required by stoichiometry). The ethylene can then be transformed into ethane and may thereby cause a significant rise in temperature, which requires immediate depressurizing of the reactor to prevent an explosion.
In the case of treatment of the C2 cut alone, polymerization of the acetylene and progressive deactivation of the catalyst may occur, because of the large concentration of unsaturated hydrocarbons in the cut to be treated, which necessitates regeneration or periodic replacement of the catalyst charge. Generally, a reserve reactor is installed to avoid interrupting production. In addition, it is necessary to use a purified hydrogen current for the reaction, and these two aspects tend to increase the investments for reserve equipment or the equipment used only for the purpose described.
The present invention overcomes the disadvantages of the known previous techniques by purification of the ethylene-rich fraction at an intermediate stage of the process.
Thus the invention concerns, according to one of its aspects, a process for fractionation of a large anhydrous gas resulting from pyrolysis of hydrocarbons containing hydrogen and hydrocarbons, particularly C1 to C3 hydrocarbons, including ethylene, propylene, and acetylene, and at least one current enriched with hydrogen and/or methane, at least one current enriched with ethylene and poor in acetylene, and at least one propylene-rich current, including stages wherein:
a) the gas resulting from the pyrolysis of hydrocarbons under pressure is cooled and liquefied progressively by passage into a series of increasingly colder heat-exchange zones. At least one condensate is separated from the pyrolysis gas after passage into each heat-exchange zone, at least one of the condensates being propylene-enriched and at least one other condensate being ethylene and ethane enriched, and containing in solution a smaller proportion of hydrogen, methane, and acetylene, and the residual hydrogen-rich gas is collected;
b) at least part of the ethylene- and ethane-enriched condensate and the propylene-enriched condensate is evaporated by a decrease in pressure. They are reheated, independently or not, in at least one of the heat-exchange zones by thermal exchange with fluids to be cooled, including at least the gas resulting from the pyrolysis, to provide, respectively, a fraction that is at least partly evaporated due to the reduction in pressure and reheating of the ethylene- and ethane-enriched fraction, and a fraction that is at least partly evaporated due to the reduction in pressure and the reheating of the propylene-enriched fraction, to provide at least part of the cold needed for cooling and for progressive liquefaction of at least said gas resulting from the pyrolysis of hydrocarbons upon passage into said successive heat-exchange zones;
c) the fractions which are at least partly evaporated, resulting from stage (b), are introduced into part of a distillation column called a de-ethanizer, the ethane- and ethylene-rich partly evaporated condensate being admitted into a point of the part of the distillation column called the de-ethanizer, higher than the propylene-enriched partly evaporated condensate, the part of the distillation column called the de-ethanizer operating under conditions of temperature and pressure allowing the separation, in an upper part, of a first current of ethylene- and ethane-enriched head gas containing, in a smaller proportion, acetylene, hydrogen, and methane, and in a lower part, a first bottom current of propylene-enriched fluid, which is collected;
d) the first current of ethylene- and ethane-enriched head gas from stage (c) in a zone of acetylene elimination by extraction with solvent and/or by selective hydrogenation of the acetylene by means of hydrogen containing in the first gaseous head current, to provide a current essentially devoid of acetylene, and
e) in the part of the distillation column called the de-methanizer, the gas current which is essentially devoid of acetylene from stage (d) is cooled and fractionated in a second hydrogen- and/or methane-enriched head gas fraction, which is collected, and a second bottom liquid fraction which is enriched with ethylene and ethane, and is essentially devoid of acetylene, and which is also collected.
The charge gas is generally essentially free of water to prevent deposits of ice in the low-temperature circuits. Thus, a water content lower than 10 ppm by volume, preferably less than 1 ppm, is desirable.
According to one of its aspects, the process according to the invention may use the gas current from the pyrolysis of hydrocarbons at a pressure of 15-50 bar, preferably 28-38 bar, and the distillation zone called the de-ethanizer may be at a pressure of 10-30 bar, preferably 14-24 bar, lower than the pressure of the pyrolysis gas.
According to one of its aspects, the process according to the invention may use evaporated condensates introduced into the part of the distillation column called the de-ethanizer; these condensates contain dissolved hydrogen in a proportion such that the first head gas current contains 2 to 10%, preferably 4 to 5%, in moles, of hydrogen, and stage (d) may be implemented by essentially ethylene-selective hydrogenation of the acetylene contained in the first head gas current by means of the hydrogen contained in the first head gas current of stage (c), the temperature of the hydrogenation zone being between 0 and 160xc2x0 C., inclusive.
According to one of its aspects, the process according to the invention may use the hydrogen dissolved in the evaporated condensates introduced into the part of the distillation column called the de-ethanizer, so that it is the only hydrogen used for the hydrogenation carried out in stage (d).
According to one of its aspects, the process according to the invention may be implemented by sending into the top of the de-ethanizer of stage (c) two or three condensates obtained after successive passage of the pyrolysis gas, respectively, into two or three last heat-exchange zones of stage (a), considering that the first heat-exchange zone is the one which is first to be in contact with the pyrolysis gas.
The pyrolysis gas may be, for example, a naphtha or ethane pyrolysis gas.
According to one of the aspects of the process according to the invention, the second head gas fraction from the de-methanizer may be purified by distillation to recover ethylene and ethane.
According to one of the aspects of the process according to the invention, the pyrolysis gas may be an ethane pyrolysis gas, or a gas for pyrolysis of the ethane/propane mixture, and the second head gas fraction from the de-methanizer may be mixed with the pyrolysis gas without ethylene recovery, for a new treatment in mixture with the pyrolysis gas in stage (a).
According to one of the aspects of the process according to the invention, the hydrogen content of the first head gas current from the de-ethanizer may be increased by the addition of hydrogen from the head of a fluid separator, this fluid coming from the cooling in a heat-exchange zone of the residual gaseous fluid resulting from refrigeration in the successive heat-exchange zones of the pyrolysis gas.
According to one of the aspects of the process according to the invention, part of the second bottom liquid fraction from the de-methanizer is recycled into the de-ethanizer, to reduce the acetylene concentration of the first head gas current from the de-ethanizer.
According to one of the aspects of the process according to the invention, stage (d) can be carried out by extraction of the acetylene by means of a solvent.
According to one of the aspects of the process according to the invention, the carbon monoxide concentration contained in the first head gas current may have a moderating effect on the catalyzed reaction rate in the acetylene elimination zone.
According to another of its aspects, this invention concerns an installation for the fractionation of a gas resulting from pyrolysis of hydrocarbons containing hydrogen and hydrocarbons, particularly C1 to C3 hydrocarbons, including ethylene, propylene, and acetylene, and at least one current enriched with hydrogen and/or methane, at least one current enriched with ethylene and poor in acetylene, and at least one current enriched with propylene, comprising:
a) means for progressively cooling and liquefying the gas from the pyrolysis of hydrocarbons under pressure by passage into a series of increasingly colder heat-exchange zones, means for separating from the pyrolysis gas at least one condensate after passage into each heat-exchange zone, at least one of the condensates being propylene-rich and at least one other condensate being ethylene- and ethane-rich, and containing in solution a smaller proportion of hydrogen, methane, and acetylene, and means for collecting the hydrogen-rich uncondensed residual gas;
b) means to evaporate, at least in part, by reduction in pressure, the ethylene and ethane-rich condensate and the propylene-rich condensate, and means to heat them independently in at least one of the heat-exchange zones by thermal exchange with the fluids to be cooled, to provide, respectively, a fraction that is at least partly evaporated due to the reduction in pressure and the heating of the ethylene- and ethane-rich fraction, and a fraction at least partly evaporated due to the reduction in pressure and the heating of the propylene-rich fraction, to provide at least part of the cold necessary for progressive cooling and liquefaction of at least the gas resulting from the pyrolysis of hydrocarbons upon its successive passage into the heat-exchange zones,
c) means to introduce the partly evaporated fractions resulting from stage (b) into part of a distillation column called the de-ethanizer, the partially evaporated ethylene- and ethane-rich condensate being admitted at a point of the part of the distillation column higher than the partly evaporated propylene-rich condensate, the part of the distillation column operating under conditions of temperature and pressure permitting the separation, in an upper part, of a first ethylene- and ethane-rich head gas current containing, in a lower proportion, acetylene, hydrogen, and methane, and in a lower part, a first propylene-rich liquid bottom current, which is collected,
d) means to send the first ethylene- and ethane-rich head gas current resulting from stage (c) into an zone for acetylene elimination by extraction with a solvent and/or by selective hydrogenation of acetylene by means of the hydrogen contained in the first head gas current, to provide an essentially acetylene-free current, and
e) means of cooling and fractionation, in part of a distillation column called the de-methanizer, of the essentially acetylene-free gas current from stage (d) and a second head gas fraction, rich in hydrogen and/or methane, which is collected, and a second ethylene- and ethane-rich bottom liquid fraction, essentially free of acetylene, which is also collected.